116 
NATURE 
[Wov. 30, 1882 
The only phases dealt with by either Halley's or Delisle’s method 
are the external and internal contacts, both at ingress and at 
egress. Delisle’s method consists in observing the times of 
contact at stations grouped about the regions where either ingress 
or egress is soonest and latest visible. The longitudes of the 
stations must be well determined, and then by combining them 
with the observed times of contact the rate at which the shadow 
cone sweeps over the earth becomes known, and from it the 
solar parallax results. At many of the sta'ions best suited for 
Delisle’s method, only the beginning or only the ending of the 
transit will be visible; but for the application of Halley’s 
method, both the beginning and the ending must be seen. The 
theory of the latter method is so complicated that it is difficult 
to explain it briefly and at the same time accurately ; but the 
the following considerations will :urfice to indicate its nature. 
The duration of a transit at any point on the earth’s surface 
depends partly upon the length of path, and partly upon the 
velocity, of that point while within the shadow cone, The 
length of path is affected by the latitude of the point, and the 
velocity by the earth’s diurnal motion, which in some regions 
accelerates, and in others retards, the progress of the shadow. 
The result is that throughout one-half the earth’s surface the 
duration of the transit is lengthened, while throughout the other 
half it is shortened ; the maximum lengthening and shi rtening 
occurring at the respective pr les of the hemispheres in questicn. 
Although these poles are not situcted at the extremities of the 
earth’s axis, it usually happens that one of them is shrouded in 
night ; but upon the sunlit side of the earth, from which alone 
observations can be made, localities may exist at some of which 
the duration of the transit will be twenty minutes or more 
greater than at others. This inequality is the condition upon 
which Halley’s method depends, and when such localities are 
accessible it may be advantageously applied. Briefly then, 
Halley’s method consists in observing the duration of a transit 
at two or more stations so selected as to give durations of 
widely different lengths; while Delisle’s method consists in 
emplcying a common standard time to note the instant when 
the transit begins, or ends, at two or more stations so chosen 
as to give very different values for that instant. 
The transit of 1761 was visible throughout Europe and was 
well observed by astronomers in all parts of that continent. 
Besides this, England sent expeditions to St. Helena and to the 
Cape of Good Hope; and English astronomers ob:erved at 
Madras and Calcutta; French astronomers were sent to Tobolsk, 
Rodriguez, and Pondicherry ; Russians to the confines of Tartary 
and China ; and Swedes to Lapland. No Jess than 117 stations 
were occupied by 176 observers ; and of these, 137 published 
their observations. When this mass of data was submitted to 
computation, the result was far from satisfactory. Values of 
the solar parallax were obtained ranging from 8.49 seconds to 
10,10 seconds ; and in their disappointment the astronomers of 
the eighteenth century concluded that too much reliance had been 
placed upon Delisle’s method. 
The transit of 1769 drew on apace ; and, to avoida repetition 
of the fancied mistake of 1761, attention was directed almost 
exclusively to Halley’s method. The conditicns of the transit 
were carefully discussed by Hornsby in England, and by Lalande 
and Pingré in France ; and it was fcund that its duration would 
be greatest in Lapland and Kamschatka, and least in the Pacific 
Ocean, California and Mexico. Astronomers were dispatched 
to all these regions. England sent the famous Capt. Cook to 
Otaheite, France sent Chappe to California; the King of 
Denmark sent Father Hell to Lapiand; and in addition 
numerous observations were made in Europe, North America, 
China, and the East Indies. The preparations were most 
elaborate, and the result better than in 1761, but still not 
satisfactory. The black drop and other distortions disturbed 
the contacts in this transit as they had done in the previous one, 
and the values of the parallax deduced by the best computers 
ranged from 8.43 seconds to 8.85 seconds. 
Thus the matter rested till 1825 and 1827 when Encke 
published abstracts of his discussion of the transits of 1761 and 
1769, from Which he deduced a parallax of 8.58 seconds. This 
discussion was not printed in full till 1835, when it immediately 
commanded the attention of astronomers, and its result, which 
Encke had modified to 8.57 seconds, was universally accepted 
for more thana quarter of a century. As time wore on, certain 
gravitational investigations led to a strong suspicion that the 
san’s distance had been over-estimated by at least three million 
miles, and the observations of Mars at its opposition in 1862 
converted this suspicion into a conviction, The eighteenth 
century transits were again rediscussed and a parallax of 8.83 
seconds was found from them by Powalky in 1865, and 8.91 
seconds by Mr. E. J. Stone in 1868. Newcomb’s paper, in 
1867, also produced a marked impression. 
The transit of 1874 was then approaching, and in the dis- 
cussion as to how it should be utilized Halley’s and Delisle’s 
methods once more played a trominent part. It was recognized 
that the uncertainty in the observed times of ¢ ntact of the 
eighteenth century transits was largely due to the black drop, 
and the causes of that phenomenon were carefully considered. 
Among them, most astronomers believed that irradiation played 
an important, if not the principal, part; but at the same time 
there was a general feeling that the telescopes of a century ago 
were bad, and that the magnificent instruments of the present 
day would give better results. In view of all the circumstances 
it was determined that the contacts should be observed with 
equatorially-mounted achromatic telescopes of from 4 to 6 inches” 
aperture or with reflectors of not less than 7 inches’ aperture, 
and that magnifying powers of from 150 to 200 diameters should 
be employed. The Germans and Russians adopted heliometers 
of about three inches’ aperture for making exact determinations 
of the positions of Venus during transit, but other nations did 
not follow thar example. 
Photography, an agency undreamed of in the eighteenth 
century, was also availabl*, and all saw the desirability of 
employing it ; but there was much difference of opinion as to 
how should this be done. The European astronomers preferred 
instruments modelled upon the Kew photoheliograph, whose 
objective has 3.4 inches aperture and 50 inches focus, giving an 
image of the sun 0.482 of an inch in diameter, which is enlarged 
by a secondary magnifier to 3 93 inches. On the other hand, 
the American astronomers contended that photographs taken 
with such instruments would be affected by troublesome errors 
due to the secondary magnifier, that position angles could not be 
measured from them accurately enough to be of any use, and 
that it would be exceedingly difficult to determine the exact 
linear value of a second of arc. They advocated the use of 
horizental photoheliographs, which are free from all these dis- 
advantages ; and the instruments which they adopted had 
apertures of 5 inches, and focal distances of 384 feet, giving 
images of the sun slightly more than 4 inches in diameter, 
Notwithstanding this rad cal difference of opinion respecting 
the best form of photoheliograph, the astronomers of the old 
and new worlds were in perfect accord as to how the instruments 
should be employed. Between the first and second contacts, 
and again between the third and fourth contacts, photographs 
about five minutes square, sh wing the indentation cut by the 
planet into the sun’s limb, were to be taken at intervals of a 
few seconds ; and from these it was hoped the true times of 
contact could be deduced with great accuracy. Between the 
second and third contacts, pictures of the entire sun were to be 
taken at short intervals, and the positions of Venus relatively to 
the sun’s centre were to be obtained from them hy subsequent 
measurements. In the latter case, the photoheliograph took the 
place of a heliometer, and was superior to that instrument in its 
power of rapidly accumulating data. 
The question of instrumental outfit having becn disposed of, 
stations were selected, and parties dispatched to almost every 
available point. The United States, Enzland, France, Germany, 
Rusia, Holland,—in short, nearly all the nations of the 
civilized world,—took part in the operations. The weather 
was not altogether propitious on the day of the transit, but 
nevertheless a mass of data was accumulated which will require 
years for its thorough di-cussion. When the parties returned 
home the contact observations were firstattacked, but it was soon 
found that they were little better than those of the eighteenth 
century. The black drop, and the atmospheres of Venus and 
the Earth, had again produced a series of complicated 
phenomena, extending over many seconds of time, from 
among which it was extremely difficult to pick out the true 
contact. It was uncertain whether or not different observers 
had really recorded the same phase, and in every case that 
que-tion had to be decided before the observations could be 
used. Thus it came about that within certain rather wide 
limits the resulting parallax was unavoidably dependent upon 
the judgment of the computer, and to that extent was mere 
guesswork. Attention was next directed to the photographs, 
and soon it began to be whispered about that those taken by 
European astronomers were a failure. Even yet I am not aware 
